For preventive or therapeutic treatment of infectious diseases caused by microorganisms, various antibacterial agents have so far been developed, and drugs such as β-lactam antibiotics (penicillins, cephems, monobactams, carbapenems, and penems), aminoglycosides, quinolones, macrolides, tetracyclines, rifamycins, chloramphenicols, and phosphomycins have been practically used. However, with the increase of clinically used amount of antibacterial agents, remarkable numbers of resistant bacterial strains to these antibacterial agents have emerged, which becomes a serious problem in the treatment of infectious diseases.
Examples of problematic bacteria, which cause particularly intractable or serious infectious diseases among those caused by resistant bacteria, include Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA). Antibacterial agents effective against these bacteria have been limited so far, and it is not certain whether or not therapeutic efficacy of the currently available drugs will be expected in the future. In particular, no drug is available at present by which specifically high efficacy against resistant Pseudomonas aeruginosa can be achieved. With the increase of aged population and the popularization of sophisticated medical technologies including human organ transplantation and anti-cancer treatments, infections frequently occurring particularly in patients with reduced immunity, i.e., so-called opportunistic infections, have become an extremely serious problem in the clinical field, and under the circumstances, early developments of measures against the resistant bacteria are desired.
Recently, the presence of drug efflux pumps has recognized as a bacterial excretion mechanism of drugs through researches on resistance acquiring mechanisms of resistant bacteria. In earlier researches, a pump that specifically excretes a tetracycline antibacterial agent from bacterial cells was identified in 1980 by the group of Levy, and the discovery was noted as a major factor of the resistance to tetracycline (L. McMurry, Proc. Natl. Acad. Sci. U.S.A., 77, 3974, 1980). Furthermore, based on recent researches, the presence of multidrug-excreting drug efflux pumps was reported in Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus bacteria, Diplococcus pneumoniae, and Neisseria gonorrhoeae. Four multidrug efflux pumps have so far been reported as homological drug efflux pumps deriving from Pseudomonas aeruginosa, and they have been considered as a cause of low drug sensitivity inherent to Pseudomonas aeruginosa (K. Poole et al., J. Bacteriol., 175, 7363, 1993; K. Poole et al., M. Microbiol., 21, 713, 1996; T. Kohler et al., M. Microbiol., 23, 345, 1997; T. Mine et al., Antimicrob. Agents Chemother., 43, 415, 1999).
The drug efflux pumps of Pseudomonas aeruginosa excrete various drugs including β-lactams, tetracyclines, chloramphenicols, and quinolones, to which the drug resistance of Pseudomonas aeruginosa is attributable.
In order to overcome the problem, it will be effective to invent an antibacterial agent that has a novel structure, by which resistance acquisition due to a drug efflux pump, one of factors of resistance acquisition, can be avoided, or develop an agent for a combinational use with currently available antibacterial agents that can restore their efficacy by inhibiting functions of drug efflux pumps.